This relates generally to imaging devices, and more particularly, to imaging devices having image sensor pixels with overflow capabilities.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an image sensor includes an array of image pixels arranged in pixel rows and pixel columns. Circuitry may be coupled to each pixel column for reading out image signals from the image pixels.
Typical image pixels contain a photodiode for generating charge in response to incident light. Image pixels may also include a charge storage region for storing charge that is generated in the photodiode. Image sensors can operate using a global shutter or a rolling shutter scheme. In a global shutter, every pixel in the image sensor may simultaneously capture an image, whereas in a rolling shutter each row of pixels may sequentially capture an image.
Image sensors may be equipped with multi-exposure high dynamic range (HDR) functionality where multiple images are captured with an image sensor at different exposure times. The images are later combined into a high dynamic range image. In conventional HDR image sensors, a long-exposure image may be sampled during a first readout cycle. Memory buffers are then typically used to store the long-exposure image. While the memory buffers store the long-exposure image, a short-exposure image is generated. The short-exposure image is then sampled in a second readout cycle. After the short-exposure image is sampled, the short-exposure image and the long-exposure image are combined to form an HDR image. However, memory buffers may add additional costs to manufacturing the image sensor (e.g., increased Silicon cost). Additionally, standard HDR image sensor pixels may have other deficiencies such as unwanted saturation of the photodiode caused by bright scenes and inability to detect flickering light.
It is within this context that the embodiments herein arise.